I went through the same process recently... had 2 carbs in my possession that I had previously rebuilt that had neither the check ball nor the brass insert. I could have lost the check balls when I rebuilt them, but I doubt it. Holley tech line said that if theres no brass insert, then it must have the ball. I suspect there was a common configuration that had neither, but at this point its just a suspicion.

In my case, I had an issue with a carb that was pulling in the secondaries too fast and easy- I couldnt hold speeds with the secondaries partially open. I went to a housing with the brass insert and went 2 steps heavier on the secondary spring and the issue is solved. Wish I could tell you if it was the spring or the different housing that made the difference.